Microporous layers multilayered

Fig. 3 Cross-sectional SEM images of some typical multilayer structures (a) p++ big mesoporous, (b) n-H- —type macroporous (adapted from Ref. Ouyang et al. 2005b) thick mesoporous (p++ type) (adapted from Ref. Lo and Murphy 2009), (d) p-type microporous (pores < 10 nm) microcavity (adapted from Ref. Escorcia-Garcia et al. 2009), (e) combination of n-type meso- and macroporous layers (adapted from Ref Ouyang et al. 2005a), (f) combination of n-type macro-meso and microporous layers (adapted from Ref Ge et al. 2013)...

Figure 5.19 shows an idealized form of the adsorption isotherm for physisorption on a nonporous or macroporous solid. At low pressures the surface is only partially occupied by the gas, until at higher pressures (point B on the curve) the monolayer is filled and the isotherm reaches a plateau. This part of the isotherm, from zero pressures to the point B, is equivalent to the Langmuir isotherm. At higher pressures a second layer starts to form, followed by unrestricted multilayer formation, which is in fact equivalent to condensation, i.e. formation of a liquid layer. In the jargon of physisorption (approved by lUPAC) this is a Type II adsorption isotherm. If a system contains predominantly micropores, i.e. a zeolite or an ultrahigh surface area carbon (>1000 m g ), multilayer formation is limited by the size of the pores. 

Figure 7.42 Types of gas sorption isotherm - microporous solids are characterised by a type I isotherm. Type II corresponds to macroporous materials with point B being the point at which monolayer coverage is complete. Type III is similar to type II but with adsorbate-adsorbate interactions playing an important role. Type IV corresponds to mesoporous industrial materials with the hysteresis arising from capillary condensation. The limiting adsorption at high P/P0 is a characteristic feature. Type V is uncommon. It is related to type III with weak adsorbent-adsorbate interactions. Type VI represents multilayer adsorption onto a uniform, non-porous surface with each step size representing the layer capacity (reproduced by permission of IUPAC).

Another type of gas separation membrane is the multilayer composite structure shown in Figure 8.9. In this membrane, a finely microporous support membrane is overcoated with a thin layer of the selective polymer, which is a different material from the support. Additional layers of very permeable materials such as silicone rubber may also be applied to protect the selective layer and to seal any defects. In general it has been difficult to make composite membranes with... 

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

In monolayer adsorption all the adsorbed molecules are in contact with the surface layer of the adsorbent. In multilayer adsorption the adsorption space accommodates more than one layer of molecules so that not all adsorbed molecules are in direct contact with the surface layer of the adsorbent. In capillary condensation the residual pore space which remains after multilayer adsorption has occurred is filled with condensate separated from the gas phase by menisci. Capillary condensation is often accompanied by hysteresis. The term capillary condensation should not be used to describe micropore filling because this process does not involve the formation of liquid menisci. 

Finally, we will consider briefly the formation of multilayer thin films by layer-by-layer deposition of hydrogen-bonded polymer pairs [51,52]. In this way a multilayer structure is obtained from potentially miscible polymer pairs. The stability of these films very much depends on the presence of hydrogen bonds, and pH may be used as an external trigger to erase the layered structure [53,54] and selectively dissolve one of the components [25-27]. This procedure allows for the preparation of microporous films not unlike the nanoporous films obtained by dissolution of the hydrogen-bonded side groups from self-assembled block copolymer-based comb-shaped supramo-lecules [15,17,18]. 

Membrane systems with pore diameters in the micropore range (gas separation, nanofiltration) are not yet commercially available but are produced for development and marketing purposes by, e.g., Velterop B.V. (Enschede, Netherlands) and Media and Process Technology Inc. (Pittsburgh, USA). These systems have an a-alumina support combined with multilayered y-alumina (mesoporous) layers and a silica (microporous) separation layer. 

A thin film composite reverse osmosis membrane can be defined as a multilayer membrane in which an ultrathin semipermeable membrane layer is deposited on a preformed, finely microporous support structure. This contrasts with asymmetric reverse osmosis membranes in which both the barrier layer and the porous substructure are formed in a single-step phase inversion process and are integrally bonded. 

Winkler (144-148) has studied the proton NMR relaxation time of water on y-Al203 and found that the correlation time = 1.2 x 10 second lies between that of water (149) (3.5 x 10 second) and ice (150) (2 X second). Winkler observed two regions of behavior, one in micropores with radii less than 1000 A, and another in macropores with larger radii. A rapid exchange occurred between the various layers of water molecules in micropores. Ebert (151,152) used dielectric constant measurements to distinguish between monolayer and multilayer water coverages. 

A novel coordination network of DJL,-homocysteic acid with strontium chloride was reported by Liu and coworkers [220]. This compound exhibited an infinite microporous multilayered structure, where chloride anions are intercalated between layers to neutralize the charge. TG and powder XRD showed that the layered structure of the compoimd, below 326 C, is sust ed in the process of the reversible loss/gain of coordinated water, confirming that the network involves a robust coordination-based cationic layer fimnework but rather... 

More effective are films with microcracks or micropores as obtained with modem chromium-plating electrolytes. Furthermore, double layer or multilayer chromium films provide very stable corrosion protection. 

Microporous polymeric membrane separators are characterized by pore sizes in the micrometer scale. Microporous polymeric membrane separators are mainly made of polyethylene (PE), polypropylene (PP), and the combinations of them (PE/PP and PP/PE/PP) because of their high chemical and mechanical stabilities. According to the number of layers, they can be classified into monolayer and multilayer polymeric microporous membranes. 

To prepare multilayer membranes, another irradiation method to prepare cross-linked microporous multilayer membranes with enhanced thermal stability has been developed. It is realized by two steps. First, the polymer-blended layers, such as poly(ethylene glycol) diacrylate/poly(ethylene glycol) methyl ether acrylate are coated onto polyolefin microporous membranes. Second, the resultant membranes are irradiated to form chemically cross-linked membranes. They exhibited higher thermal and electrochemical properties compared to conventional separators. TOth the increase of irradiation dose, the thermal stability of the resultant membranes increases accordingly. By using the microporous multilayer membranes, the advantages of each component layers are well combined. 

With pervaporation membranes the water can be removed during the condensation reaction. In this case, a tubular microporous ceramic membrane supplied by ECN [124] was used. The separating layer of this membrane consists of a less than 0.5 mm film of microporous amorphous silica on the outside of a multilayer alumina support. The average pore size of this layer is 0.3-0.4 nm. After addition of the reactants, the reactor is heated to the desired temperature, the recyde of the mixture over the outside of the membrane tubes is started and a vacuum is apphed at the permeate side. In some cases a sweep gas can also be used. The pressure inside the reactor is a function of the partial vapor pressures and the reaction mixture is non-boiling. Although it can be anticipated that concentration polarization will play an important role in these systems, computational fluid dynamics calculations have shown that the membrane surface is effectively refreshed as a result of buoyancy effects [125]. 

All the empirical equations dealt with in Section 3.2 are for adsorption with monolayer coverage, with the exception of the Freundlich isotherm, which does not have a finite saturation capacity and the DR equation, which is applicable for micropore volume filling. In the adsorption of sub-critical adsorbates, molecules first adsorb onto the solid surface as a layering process, and when the pressure is sufficiently high (about 0.1 of the relative pressure) multiple layers are formed. Brunauer, Emmett and Teller are the first to develop a theory to account for this multilayer adsorption, and the range of validity of this theory is approximately between 0.05 and 0.35 times the vapor pressure. In this section we will discuss this important theory and its various versions modified by a number of workers since the publication of the BET theory in 1938. Despite the many versions, the BET equation still remains the most important equation for the characterization of mesoporous solids, mainly due to its simplicity. 

Type II isotherms are typical of physical adsorption on nonporous solids. In contrast to type I, the adsorbate molecules in these cases also have relatively strong mutual interactions, which leads to the tendency for multilayer formation. The initial rapidly rising part of the isotherm corresponds to the equivalent type I adsorption. Point B on the curve is identified with complete mono-layer coverage. Multilayer formation then begins which may lead to surface condensation. Type II isotherms are sometimes encountered for microporous solids, in which case point B would correspond to both completion of mono-layer coverage and filling of the micropores by capillary condensation. The rest of the curve would then correspond to normal multilayer formation. 

To separate contributions due to micropore filling on one hand and the formation of mono- and multilayers on the other hand which are superimposed at relative pressures below 0.2- 0.3, the f-plot or the aj-plot approach can be applied. Both methods use empirical reference isotherms to be compared with the isotherms taken for the sample under investigation. In the f-plot method, the statistical layer thickness t of a nonmicroporous material is related to the relative pressure plp. One of the most frequently used relationship for the layer thickness is the empirical Harkins-Jura equation [67] derived for metal oxides ° ... 

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